The present invention relates to a photomask for use in forming a fine pattern in fabrication of a semiconductor integrated circuit device or the like, a pattern formation method using the photomask and a mask data creation method for the photomask.
Recently, there are increasing demands for further refinement of circuit patterns for increasing the degree of integration of a large scale integrated circuit device (hereinafter referred to as the LSI) realized by using semiconductor. As a result, it has become very significant to thin an interconnect pattern included in a circuit.
Now, the thinning of an interconnect pattern by a conventional optical exposure system will be described on the assumption that positive resist process is employed. In this case, a line pattern means a portion of a resist film not exposed to exposing light, namely, a resist portion (a resist pattern) remaining after development. Also, a space pattern means a portion of the resist film exposed to the exposing light, namely, an opening portion (a resist removal pattern) formed by removing the resist film through the development. In the case where negative resist process is employed instead of the positive resist process, the definitions of the line pattern and the space pattern are replaced with each other.
When a pattern is formed by using the optical exposure system, a photomask in which a light-shielding pattern of Cr (chromium) or the like is drawn in accordance with a desired pattern on a transparent substrate (a substrate having a transparent property) of quartz or the like is conventionally used. In such a photomask, a region where the Cr pattern exists is a light-shielding portion that does not transmit exposing light of a given wavelength at all (having transmittance of substantially 0%) and a region where no Cr pattern exists (an opening) is a transparent portion that has transmittance equivalent to that of the transparent substrate against the exposing light (having transmittance of substantially 100%). At this point, all mask patterns are drawn on the transparent substrate, and in the pattern exposure, the transparent substrate is irradiated from a side where the mask patterns are not provided, and therefore, the exposing light transmits the mask patterns after transmitting the transparent substrate. Accordingly, in the following description, transmittance of a mask pattern against exposing light is not the absolute transmittance of each portion of the mask pattern but is relative transmittance obtained on the basis of the transmittance of a transparent substrate against the exposing light unless otherwise mentioned.
In the case where a photomask as described above is used for the exposure of a wafer where a resist has been applied, an image of light having passed through the mask is projected onto the wafer. In this case, a light-shielding portion of the photomask corresponds to an unexposed region of the resist and an opening (transparent portion) of the photomask corresponds to an exposed region of the resist, so that a desired resist pattern can be formed on the wafer. Accordingly, such a photomask, namely, a photomask composed of a light-shielding portion and a transparent portion against exposing light of a given wavelength, is designated as a binary mask. It is, however, difficult to accurately form a fine pattern smaller than the exposure wavelength (the wavelength of the exposing light) by using the binary mask even when a mask pattern in substantially the same shape as the desired pattern is provided on the binary mask. This is because the diffraction of light is so remarkable in the formation of a fine pattern that the light rounds also to a portion of the resist corresponding to a light-shielding portion of the photomask, and hence, sufficient contrast cannot be realized in an optical image formed on the resist. As a result, sufficient contrast in the sensitivity cannot be attained between an unexposed region and an exposed region of the resist.
In a method proposed for overcoming this problem, oblique incident illumination (off-axis illumination), that is, a kind of modified illumination, is employed in the exposure optical system, so as to improve the contrast of an optical image in formation of repeated patterns. The oblique incident illumination is effective in improving the contrast of an optical image in the formation of repeated patterns, and in particular, it can keep high contrast even in a defocus state in the formation of repeated patterns. In other words, when the oblique incident illumination is employed, the contrast (exposure margin) and depth of focus (DOF) are improved in the formation of repeated patterns.
However, the oblique incident illumination cannot improve the contrast and the DOF in formation of all patterns. In particular, it cannot exhibit any improvement effect in formation of a fine pattern smaller than the exposure wavelength and corresponding to a transparent portion present isolatedly in a light-shielding portion of a photomask. Therefore, instead of using a mask pattern in substantially the same shape as a desired pattern, a method using a mask on which a principal pattern corresponding to a desired pattern and in substantially the same shape as the desired pattern and an auxiliary pattern that does not affect the resist pattern shape but affects an optical image are both provided has been proposed (see, for example, Patent Document 1).
FIG. 29A is a plan view of a photomask disclosed in Patent Document 1. As shown in FIG. 29A, a principal pattern 900 made of an isolated transparent portion is provided in a light-shielding portion 901 of the photomask. Also, auxiliary patterns 902 made of transparent portions each having a width limited to transmit exposing light to the extent that a resist pattern is not formed are provided on both sides of the principal pattern 900. In this case, in order to improve the contrast of an optical image formed by the exposing light having passed through the principal pattern 900, it is necessary to set a distance between the center of the principal pattern 900 and the center of each auxiliary pattern 902 to a range from λ/NA to 1.4×λ/NA, wherein λ indicates a wavelength of the exposing light and NA indicates a reduction ratio of a reduction projection optical system of a projection aligner. When auxiliary patterns are provided on a photomask as shown in FIG. 29A, an optical image similar to repeated patterns is formed. Therefore, when such a photomask is subjected to exposure using the oblique incident illumination, the contrast and the focus of depth can be improved.
Although the resolution of a simple pattern including an isolated transparent portion is improved by introducing auxiliary patterns as described above, the following problem arises when auxiliary patterns are provided in a pattern of complicated layout including a plurality of principal patterns: In the case where there are a plurality of principal patterns, if an auxiliary pattern is disposed in a position for improving the contrast of an isolated single principal pattern, the contrast may be lowered on the contrary. Also, when an auxiliary pattern is to be provided to each of the plural principal patterns, some auxiliary patterns are close to each other at a distance smaller than the exposure wavelength. When the auxiliary patterns each provided so as not to singly form a resist pattern are thus close to each other, the exposing light transmits the auxiliary patterns to the extent that a corresponding portion of the resist is exposed.
As a countermeasure for this problem, the following method has been proposed (see, for example, Patent Document 2): A plurality of auxiliary patterns are provided in accordance with the layout including a plurality of principal patterns, and auxiliary patterns adjacent to each other out of the plural auxiliary patterns are set to transmit exposing light in opposite phases to each other, so that the lights respectively passing through the adjacent auxiliary patterns can cancel each other. Thus, the contrast of the light passing through the principal patterns is improved, and transfer of the auxiliary patterns is prevented.
FIG. 29B is a plan view of a photomask disclosed in Patent Document 2. As shown in FIG. 29B, a principal pattern 900 made of an isolated transparent portion is provided in a light-shielding portion 901 of the photomask. Also, auxiliary patterns 902 made of a transparent portion are provided on both sides of the principal pattern 900, and auxiliary patterns 903 made of a phase shifter are provided on sides of the auxiliary patterns 902 opposite to the principal pattern 900.
Patent Document 1: Japanese Laid-Open Patent Publication No. 5-165194
Patent Document 2: Japanese Laid-Open Patent Publication No. 2000-206667
Patent Document 3: Japanese Patent Publication No. 3708875